In the early 1900's, the first method was developed for spraying cementitious materials. The method was to place the dry ingredients into a hopper and then to blow the dry material out through a hose with compressed air while injecting water at the nozzle as it was released. Since the material was sprayed using a spray “gun”, the process came to be referred to as “gunite”. The composition was a combination of cement, sand, and gravel aggregate. This “dry mix” process was the primary process used for spraying cementitious compositions for the first half of the 20th Century. A drawback associated with gunite application using dry mix of cementitious material is the significant amount of ambient air contamination caused by dust particles being blown through the hose and failing to completely mix with the water injected at the nozzle. This resulted in a detrimental work environment for those applying the “gunite” requiring significant safety precautions to be undertaken to protect the workers thereby increasing the time and cost for using gunite. A further drawback is, due to the imperfect mixing of the dry materials and the water, gunite applications were typically associated with significant material waste due to less than desirable ability to stick to application surfaces, especially vertical or overhead surfaces.
An improvement was made in 1960's whereby a “wet mix” process was developed and used for application to substrate surfaces. This involves pumping previously prepared cementitious materials in a viscous state to a nozzle and introducing compressed air at the nozzle. The compressed air causes the viscous cementitious materials through the nozzle and onto a receiving surface. This wet mix process became known as “shotcrete”. Shotcrete applications of cementitious materials provide two advantages over gunite applications of cementitious materials. A first advantage of shotcrete applications is the reduction in rebound which is defined as the waste of material when the applied cementitious material does not stick to the surface and falls to the floor. A second advancement over gunite is the reduction of dust generated during the application process. However, while rebound has been reduced by employing the wet mix process of shotcrete, the average application of shotcrete results in between 15% and 20% loss of material resulting in an increase in cost for applying the cementitious material as well as an increase in time required to fully coat the desired substrate.
Today, shotcrete has emerged as the only acceptable industry term to correctly describe the two types of “pneumatically applied concrete” and may be used to refer to either the wet or dry process. Both processes continue to use similar formulations which include some combination of cement, sand, and one or more types of larger aggregate such as rock, gravel, etc. Other industry advances include the use of concrete admixtures such as superplasticizers and/or water reducers and accelerators. However, due to the nature of the chemical admixtures, it is common for individual admixtures to intermix with one another prior to being incorporated into the cementitious material. Premature intermixing may result in the desired effects of the admixtures from being imparted to the cementitious material. To minimize the effects of premature intermixing, the admixtures are conventionally added at the nozzle of the gun and then mixed with the cementitious material flowing therethrough. A drawback associated with this manner of combining the cementitious materials with the respective admixtures is an imperfect mixing process resulting in a cementitious material have less than optimal properties.
Conventional cementitious compositions provide a compressive strength of substantially 4,000 PSI range and have relatively low tensile and flexural strength associated therewith. Typically, tensile strength is normally in the 500-1,000 PSI range and the flexural strength is approximately 200-700 PSI. However, with a weight of substantially 146 pounds per cubic foot, it is difficult for the material to be sprayed on and adhered to either vertical substrates or overhead substrates without using some type of mesh or wire overlay that aids in adherence of the cementitious material to the substrate. A drawback associated with using these overlays is the added cost and reduced ability of the cementitious material to bond directly to the substrate.
Thus, in view of the characteristics of shotcrete application of cementitious materials, there is a significant drawback with using such applications in mining and tunneling operations. In the mining and tunneling industry, the desire and need for safety encourages the use of wire mesh and steel framing structure to be installed before the shotcrete is sprayed onto the substrate. The result is an additional installation step that increases time, labor, and cost associated with the application.
Consequently, a need exists to provide a cementitious composition that can be applied by a sprayer which can minimize the above discussed drawbacks. A cementitious composition according to invention principles addresses the known deficiencies and drawbacks.